diameter_app

Callback module of a Diameter application.

A diameter service as started by diameter:start_service/2
configures one of more Diameter applications, each of whose
configuration specifies a callback that handles messages specific to
the application.
The messages and AVPs of the application are defined in a
dictionary file whose format is documented in
diameter_dict(4) while the callback module is documented here.
The callback module implements the Diameter application-specific
functionality of a service.

A callback module must export all of the functions documented below.
The functions themselves are of three distinct flavours:

peer_up/3 and peer_up/3 signal the
attainment or loss of connectivity with a Diameter peer.

Note!

The arities given for the the callback functions here assume no extra
arguments.
All functions will also be passed any extra arguments configured with
the callback module itself when calling diameter:start_service/2
and, for the call-specific callbacks, any extra arguments passed to
diameter:call/4.

DATA TYPES

capabilities() = #diameter_caps{}

A record containing the identities of
the local Diameter node and the remote Diameter peer having an
established transport
connection, as well as the capabilities as
determined by capabilities exchange.
Each field of the record is a 2-tuple consisting of
values for the (local) host and (remote) peer.
Optional or possibly multiple values are encoded as lists of values,
mandatory values as the bare value.

The state maintained by the application callback functions
peer_up/3, peer_up/3 and (optionally) pick_peer/4.
The initial state is configured in the call to
diameter:start_service/2 that configures the application on a service.
Callback functions returning a state are evaluated in a common
service-specific process while
those not returning state are evaluated in a request-specific
process.

Mod:peer_up(SvcName, Peer, State) -> NewState

Invoked to signal the availability of a peer connection.
In particular, capabilities exchange with the peer has indicated
support for the application in question, the RFC 3539 watchdog state
machine for the connection has reached state OKAY and Diameter
messages can be both sent and received.

Note!

A watchdog state machine can reach state OKAY from state
SUSPECT without a new capabilities exchange taking place.
A new transport connection (and capabilities exchange) results in a
new peer_ref().

Note!

There is no requirement that a callback return before incoming
requests are received: handle_request/3 callbacks must be
handled independently of peer_up/3 and peer_up/3.

Mod:peer_down(SvcName, Peer, State) -> NewState

Invoked to signal that a peer connection is no longer available
following a previous call to peer_up/3.
In particular, that the RFC 3539 watchdog state machine for the
connection has left state OKAY and the peer will no longer be a
candidate in pick_peer/4 callbacks.

Invoked as a consequence of a call to diameter:call/4 to select a destination
peer for an outgoing request.
The return value indicates the selected peer.

The candidate list contains only those peers that have advertised
support for the Diameter application in question during capabilities
exchange, that have not be excluded by a filter option in
the call to diameter:call/4
and whose watchdog state machine is in the OKAY state.
The order of the elements is unspecified except that any
peers whose Origin-Host and Origin-Realm matches that of the
outgoing request (in the sense of a {filter, {all, [host, realm]}}
option to diameter:call/4)
will be placed at the head of the list.

A callback that returns a peer() will be followed by a
prepare_request/3
callback and, if the latter indicates that the request should be sent,
by either handle_answer/4
or handle_error/4 depending
on whether or not an answer message is received from the peer.
If the transport becomes unavailable after prepare_request/3 then a
new pick_peer/4 callback may take place to
failover to an alternate peer, after which prepare_retransmit/3 takes the
place of prepare_request/3 in resending the
request.
There is no guarantee that a pick_peer/4 callback to select
an alternate peer will be followed by any additional callbacks since a
retransmission to an alternate peer is abandoned if an answer is
received from a previously selected peer.

Returning false or {false, NewState} causes {error,
no_connection} to be returned from diameter:call/4.

Note!

The return value {Peer, NewState} is only allowed if
the Diameter application in question was configured with the
diameter:application_opt(){call_mutates_state, true}.
Otherwise, the State argument is always
the intial value as configured on the application, not any subsequent
value returned by a peer_up/3
or peer_up/3 callback.

Invoked to return a request for encoding and transport.
Allows the sender to use the selected peer's capabilities
to modify the outgoing request.
Many implementations may simply want to return {send, Packet}

A returned packet() should set the
request to be encoded in its
msg field and can set the transport_data field in order
to pass information to the transport process.
Extra arguments passed to diameter:call/4 can be used to
communicate transport (or any other) data to the callback.

A returned packet() can set
the header field to a
#diameter_header{} to specify values that should
be preserved in the outgoing request, values otherwise being those in
the header record contained in Packet.
A returned length, cmd_code or application_id is
ignored.

A returned PostF will be evaluated on any encoded
#diameter_packet{} prior to transmission, the bin field
containing the encoded binary.
The return value is ignored.

Returning {discard, Reason} causes the request to be aborted
and the diameter:call/4 for which the
callback has taken place to return {error, Reason}.
Returning discard is equivalent to returning {discard,
discarded}.

Invoked to return a request for encoding and retransmission.
Has the same role as prepare_request/3 in the case that
a peer connection is lost an an alternate peer selected but the
argument packet() is as returned
by the initial prepare_request/3.

Returning {discard, Reason} causes the request to be aborted
and a handle_error/4 callback to
take place with Reason as initial argument.
Returning discard is equivalent to returning {discard,
discarded}.

By default, an incoming answer message that cannot be successfully
decoded causes the request process to fail, causing
diameter:call/4
to return {error, failure} unless the detach option was
specified.
In particular, there is no handle_error/4 callback in this
case.
The diameter:application_opt()answer_errors can be set to change this behaviour.

Mod:handle_error(Reason, Request, SvcName, Peer) -> Result

Invoked when an error occurs before an answer message is received
in response to an outgoing request.
The return value is returned from diameter:call/4 unless the
detach option was specified.

Reason timeout indicates that an answer message has not been
received within the time specified with the corresponding diameter:call_opt().
Reason failover indicates
that the transport connection to the peer to which the request has
been sent has become unavailable and that not alternate peer was
not selected.

Invoked when a request message is received from a peer.
The application in which the callback takes place (that is, the
callback module as configured with diameter:start_service/2)
is determined by the Application Identifier in the header of the
incoming request message, the selected module being the one
whose corresponding dictionary declares
itself as defining either the application in question or the Relay
application.

The msg field will be undefined in case the request has
been received in the relay application.
Otherwise it contains the record representing the request as outlined
in diameter_dict(4).

The errors field specifies any Result-Code's identifying errors
that were encountered in decoding the request.
In this case diameter will set both Result-Code and
Failed-AVP AVP's in a returned
answer message() before sending it to the peer:
the returned message() need only set any other required AVP's.
Note that the errors detected by diameter are all of the 5xxx series
(Permanent Failures).
The errors list is empty if the request has been received in
the relay application.

The transport_data field contains an arbitrary term passed into
diameter from the transport module in question, or the atom
undefined if the transport specified no data.
The term is preserved if a message() is returned but must be set
explicitly in a returned packet().

Send the specified answer message to the peer.
In the case of a packet(), the
message to be sent must be set in the
msg field and the header field can be set to a
#diameter_header{} to specify values that should be
preserved in the outgoing answer, appropriate values otherwise
being set by diameter.

{protocol_error, 3000..3999}

Send an answer message to the peer containing the specified
protocol error.
Equivalent to

{reply, ['answer-message' | Avps]

where Avps sets the Origin-Host, Origin-Realm, the specified
Result-Code and (if the request sent one) Session-Id AVP's.

Note that RFC 6733 mandates that only answers with a 3xxx series
Result-Code (protocol errors) may set the E bit.
Returning a non-3xxx value in a protocol_error tuple
will cause the request process in question to fail.

{relay, Opts}

Relay a request to another peer in the role of a Diameter relay agent.
If a routing loop is detected then the request is answered with
3005 (DIAMETER_LOOP_DETECTED).
Otherwise a Route-Record AVP (containing the sending peer's Origin-Host) is
added to the request and pick_peer/4
and subsequent callbacks take place just as if diameter:call/4 had been called
explicitly.
The End-to-End Identifier of the incoming request is preserved in the
header of the relayed request.

The returned Opts should not specify detach.
A subsequent handle_answer/4
callback for the relayed request must return its first
argument, the #diameter_packet{} record containing the answer
message.
Note that the extra option can be specified to supply arguments
that can distinguish the relay case from others if so desired.
Any other return value (for example, from a
handle_error/4 callback)
causes the request to be answered with 3002 (DIAMETER_UNABLE_TO_DELIVER).

discard

Discard the request.
No answer message is sent to the peer.

{eval, Action, PostF}

Handle the request as if Action has been returned and then
evaluate PostF in the request process.
The return value is ignored.

{eval_packet, Action, PostF}

Like eval but evaluate PostF on any encoded
#diameter_packet{} prior to transmission, the bin field
containing the encoded binary.
The return value is ignored.

Note that protocol errors detected by diameter will result in an
answer message without handle_request/3 being invoked.